Prepping for Paint on Project RamRodder

When building a street rod, few things are more disappointing than working hard and spending money on some aspect of its construction only to have the results fall far short of your expectations. In some cases minor flaws are only obvious to those who know their cars intimately and will never be noticed by anyone else, but then there are those that seem like they could be seen from space—and those are usually in the paintwork, or more accurately the surface below.

While it's obvious that laying down paint is more complicated than pointing the spray gun the general direction of the car and pulling the trigger, regardless of how well paint is applied it will never make bad bodywork better. Paint a corncob and it's still going to look rough, the reason is the paint will not level the surface but follow it. Those spectacular paintjobs we've all seen on the pages of Street Rodder and in person look that way because they are extremely reflective and a smooth surface is more reflective than a rough one. And block sanding is the secret to a flat surface.

Block sanding is a term that most of those in the hobby have heard, however it is still confused with hand sanding, which should be avoided. The problem with hand sanding is that pressure is unevenly applied to the sandpaper; look at a handprint on a flat surface to see what we're talking about. (Editor's note: The staff agrees with Ron as his hand print oddly resembles a paw.) We've said this before but fingers are for scratching, hands are for holding, and blocks are for sanding.

One of the common questions about sanding in general is what grit sandpaper to use. Coarse grits labeled in the 36-80 range are used for removing old paint, filler, and rust. Grits between 100 and 320 are used to smooth body filler and primer surfacer. To prepare primer for paint, 400- to 600-grit abrasives will smooth the surface while still giving the paint something to "bite" into. Finally, extremely fine grits, such as 1,000 and up, are for color sanding prior to buffing.

Along with grit, two additional terms that will be used to describe sandpaper are open coat and closed coat. Basically this describes how tightly packed the grit is on the backing material. Open coat abrasives have the grit spaced further apart, which helps reduce clogging when soft materials are being sanded. Closed coat means that the grit is packet tightly together and all of the backing is covered. Closed coat sandpaper is more aggressive simply because there is more grit on the backing.

2/231. After a thorough sanding, Jake Brazille shot the bare body of our Plymouth with PPG’s DP90LF epoxy primer, followed by two coats of their D839 Primer Surfacer to build the surface for block sanding.

3/232. Our well-used sanding blocks came in kit form from Eastwood; the variety of sizes provide the right tool for most situations.

4/233. There are some occasions when it’s necessary to improvise. We’ve used paint sticks, wood dowels, and pieces of rubber hose to sand challenging areas.

5/234. Brazille used the longest block possible on the areas being sanded. Rather than sand in a straight line, try for a cross-hatch pattern.

6/235. With the block held at a slight angle, it’s pushed away at an angle.

7/236. After sanding in one direction a second pass is made at a right angle to the first.

8/237. While the Plymouth’s top looked good, Brazille’s block sanding technique revealed signs of work done at some time in the coupe’s past.

9/238. Although most flaws are obvious when block sanding, using a different color primer as a guidecoat will emphasize imperfections.

10/239. The area behind the passenger door was guidecoated and then sanded.

11/2310. With the guidecoat sanded off, anything that remains, like this area being pointed to, requires attention. We hadn’t noticed it before the guidecoat.

12/2311. Our Plymouth has lots of compound curves that are a challenge to block sand. Sandpaper wrapped around a paint stick worked well in the area adjacent to the rear window.

Another question is should the surface be dry or wet sanded? The best thing to do is to follow the manufacturer's recommendations but generally wet sanding polyester primers is not suggested. These materials may allow moisture to penetrate the surface, which can create problems later on. In our case Jake Brazille, of Jake's Place in Florence, Oregon, applied DP90LF epoxy primer followed by D839 Primer Surfacer, which may be dry or wet sanded, depending on preference.

For D839 PPG recommends the following:
For dry sanding: 240 followed by 400-500 grit
For wet sanding: 400 followed by 600-800-grit.

We found that D839 dry sands extremely well without clogging the sandpaper so there was no need to make the mess that comes along with wet sanding, at least not yet.

Our first round of block sanding was done with the longest blocks that would work in a given area using 240-grit sandpaper to make the first cut quickly. Our Plymouth's body shell has very few flat surfaces with compound curves galore, so a variety of blocks were used. We followed the contour of the sheetmetal, sanding at slight angles to establish a crisscross pattern.

In a few areas we discovered some minor filling would be required. Those areas were sanded with a dual-action sander and a thin coat of filler was applied. As D839 is to be applied over an adhesion primer any bare metal was spot primed with DP90LF before more primer surfacer was applied.

After the necessary repairs were made, the next step was wet sanding the body with 400-grit wet/dry sandpaper. We used a spray bottle to wet the surface and occasionally dipped the sanding block into a bucket of water to keep the sandpaper from clogging. The area being sanded was wiped off with a damp sponge to remove any residue that accumulated.

With no new faults found, the final step in getting the body ready for paint was one more round of wet sanding block sanding with blocks and 600-grit paper. At that point our Plymouth's body was as smooth as a cue ball and ready for paint for color. It also meant that we had the hood, doors, fenders, and a trunklid full of louvers to work on next and that means there are more block parties to come.

13/2312. It’s hard to tell but this area of the quarter-panel has a reverse curve where it meets the fender. The round Eastwood Durablock was the perfect match for the shape.

14/2313. Another challenging area on many cars is the cowl. Some portions are flat, others are curved so a combination of blocks are required.

15/2314. Blocking the roof revealed an area that needed some additional bodywork (arrows).

16/2321. We continue repairing the vehicle body spots as needed.

17/2315. To get down to bare metal in those areas needing further attention, 180-grit paper was used on a DA sander.

18/2322. With all the necessary repairs made the entire body was block sanded, re-primed with D839 as necessary, and then wet sanded in preparation of color.

19/2316. It seems counterproductive to sand off all the primer we’d applied, but sometimes that’s what it takes and we’re sure the end result will be worth all the effort.

20/2317. All the little dark spots are slightly low. Almost invisible in primer, block sanding made them obvious.

21/2318. Rather than metalworking the area we elected to fill the imperfections.

22/2319. A thin coat of filler was applied that extended well past the damaged area to allow for featheredging.

23/2320. The filler was block sanded, which put most of it on the shop floor but made the top perfectly smooth.